feat: enhance authentication and publication download tracking

- Added JWT authentication support with configurable secret and expiration.
- Introduced optional API key validation for endpoints.
- Implemented tracking of publication downloads by researchers, storing records in a new PublicationDownload model.
- Updated export endpoints to conditionally register downloads based on user authentication.
- Enhanced researcher search response to indicate if publications were downloaded by the current user.
- Updated environment configuration to include new JWT settings.

backend/.env.example

@@ -7,4 +7,13 @@ API_KEY_VALUE=123412341234
 DATABASE_URL=postgresql://postgres:postgres@db:5432/orcid_db
 REDIS_URL=redis://redis:6379/0
 
-BASE_URL=http://localhost:8000/api
+BASE_URL=http://localhost:8000/api
+
+# JWT (login ORCID)
+JWT_SECRET=change_me
+JWT_ALGORITHM=HS256
+JWT_EXPIRES_MINUTES=720
+
+# ORCID OAuth 3-legged (authorization code)
+# Debe coincidir exactamente con el redirect URI configurado en tu app ORCID.
+ORCID_REDIRECT_URI=http://localhost:8000/api/auth/orcid/callback